1. Field of the Invention
This invention relates to an injection mold, and more particularly to an injection mold which is for injection-molding a tubular glass-like article having a bottom at one end and opening at the other end and has a gate open to the cavity at a portion defining the bottom of the article.
2. Description of the Prior art
In injection molding of a plastic article, the injection mold is generally cooled by flowing mold-cooling water through the mold in order to quickly solidify molten resin injected into the cavity of the mold, thereby shortening the molding cycle. In the case of a mold for molding a glass-like article having a bottom at one end, a blind hole is formed in the core for defining the inner surface of the molded article, and a cooling channel for cooling the core is formed in the blind hole by providing partitioning, a pipe (bayonet), a threaded pipe or the like in the blind hole. For example, such an injection mold is disclosed in Japanese Unexamined Patent Publication No. 62(1987)-62721.
In the conventional injection mold having such a cooling system, there has been the following problem. That is, since the gate through which injection material is injected into the cavity is generally formed in the cavity plate at a portion defining the bottom of the article, a large injection pressure acts on the end face of the core. Existence of the blind hole weakens the strength of the end portion of the core except the peripheral portion. Accordingly, in order to withstand the injection pressure, the end portion of the core must have a substantial thickness. However, when the thickness of the end portion of the core is large, the distance between the end surface of the core and the heat exchange surface between the cooling water and the core is enlarged, and accordingly, the end surface of the core which is most heated by molten resin injected through the gate cannot be sufficiently cooled in a short time, whereby the molding cycle is elongated. That is, if the mold is opened before the injected resin is sufficiently cooled and solidified at the bottom of the article, injection material can string and/or a sink mark can be produced in the bottom of the article.